Lawn and garden equipment employing internal combustion engines above three or four horsepower are often offered with starting equipment options. For instance, such engines normally include a recoil rope starter for permitting manual engine starting. Additionally, an electric starter motor option is often available wherein 110 volt AC house current may be employed to energize a starter motor to crank the engine for starting purposes. Electric starters permit women and children to use heavier horsepower equipment than otherwise possible, and electric starters are popular with "walk behind" snow blowers in that the manual cranking of a snow blower engine in cold weather is difficult.
In that lawn and garden equipment often becomes wet, and snow blowers often accumulate snow thereon, it is desirable that the starter motor and associated switch structure be electrically insulated as highly as possible to protect the user against electrical shock, and while tools such as hand power drills, hedge trimmers, and the like have long been doubly insulated, such electrical protective constructions have not commonly been used in starter motors for internal combustion engines.
Starter motors for small internal combustion engines normally include an armature shaft having a cantilevered portion extending from the motor casing upon which Bendix type starter structure is mounted, as shown in the assignee's U.S. Pat. No. 3,791,684. As the starter structure includes threads cooperating with nut apparatus which is axially translated by the motor shaft rotation it is necessary to harden the cantilever portion of the shaft for wear-resistant purposes. Accordingly, it has been necessary to heat treat the entire armature and starter shaft even though the portion of the shaft upon which the starter structure is not mounted does not require hardening and the shaft heat treatment often distorts the shaft requiring time consuming and expensive straightening.
In the electric engine starters of the aforementioned type the Bendix type starter structure may include a starter pinion rotatively mounted upon a nut member and a frictional driving relationship is produced between the gear and nut member by an elastomeric material on the nut member which frictionally engages the gear. As the torque transmission of the gear increases the frictional engagement between the elastic material and gear increases, and the elastic material will produce a cushioning effect both during cranking, and during initial engagement between the pinion gear and the engine flywheel teeth. The presence of a cushioning elastomeric member between the starter pinion and starter nut is shown in the assignee's U.S. Pat. No. 3,791,685 and the instant invention presents improvements over this prior teaching.
The electric starter motor is operated by switch structure associated with the electrical conductors for interconnecting the motor with the household 110 volt AC electrical supply. The switch structure of prior art electrical starter motors for use with lawn and garden equipment has not been as watertight and protective against the possibility of electrical shock as desired, and as the switch structure, as well as the starter engine structure, provides the possibility for electrical shock, and these components, together, contribute to the safety of the starter apparatus, it is important that the starter switch as well as the starter motor casing be capable of meeting high dielectric safety requirements.